In a fabrication process of a semiconductor device, a polishing app widely used for polishing a surface of a wafer. The polishing apparatus of this type includes a polishing table for supporting a polishing pad having a polishing surface, a substrate holder, which is called a top ring or a polishing head, for holding the wafer, and a polishing liquid supply nozzle for supplying a polishing liquid onto the polishing surface.
The polishing apparatus polishes the wafer as follows. The polishing table is rotated together with the polishing pad, while the polishing liquid is supplied from the polishing liquid supply nozzle onto the polishing surface. The wafer is held by the substrate holder, and the wafer is rotated about its axis. In this state, the substrate holder presses the surface of the wafer against the polishing surface of the polishing pad so that the surface of the wafer is placed in sliding contact with the polishing surface in the presence of the polishing liquid. The surface of the wafer is planarized by a mechanical action of abrasive grains contained in the polishing liquid and a chemical action of the polishing liquid. Such polishing apparatus is called a CMP (chemical mechanical polishing) apparatus.
During polishing of the wafer, a frictional force acts on the wafer, because the surface of the wafer is in sliding contact with the polishing pad. Therefore, in order to prevent the wafer from disengaging from the substrate holder during polishing of the wafer, the substrate holder includes a retaining ring. This retaining ring is disposed so as to surround the wafer, and is configured to press the polishing pad outside the wafer.
A polishing rate (which is also referred to as a removal rate) of the wafer can vary depending on polishing conditions, such as a load of the wafer on the polishing pad, a load of the retaining ring, rotating speeds of the polishing table and the wafer, and type of polishing liquid. In a case of polishing a plurality of wafers successively, the polishing conditions are typically kept constant in order to obtain the same polishing results. However, as the wafers are polished, an edge profile of each wafer may vary gradually despite the same polishing conditions. Specifically, the polishing rate of an edge portion of the wafer increases in accordance with an increase in the number of polished wafers.
A possible cause of such an increase in the polishing rate is a deformation of the retaining ring. FIG. 19 is a schematic view showing the retaining ring when polishing of a wafer is performed. As shown in FIG. 19, during polishing of a wafer W, a retaining ring 200 is pressed against a polishing surface 201a of a rotating polishing pad 201. As a result, the retaining ring 200 wears. In particular, an inner circumferential surface and an outer circumferential surface of the retaining ring 200 wear, thus forming a rounded shape. If the inner circumferential surface of the retaining ring 200 wears as shown in FIG. 19, a force of the retaining ring 200 that presses a pad region near the edge portion of the wafer W is reduced. As a result, the polishing rate of the edge portion increases.